The present invention relates to voltage regulators. More particularly, this invention relates to circuits and methods that provide the ability to adjust the minimum peak inductor current level for Burst Mode™ (hereinafter, “Burst Mode”) in current-mode DC-DC regulators (i.e. regulators that respond to measurements of the output current).
Voltage regulators are power supply circuits that use a closed loop design to provide a predetermined and substantially constant output voltage, even while using an input voltage source which may be poorly specified or fluctuating. Furthermore, many electronic products use voltage regulators to convert an input voltage into a regulated output voltage that may be higher or lower than the input voltage. Accordingly, voltage regulators function as both a voltage converter in addition to a voltage stabilizer.
There are two major types of regulators: linear regulators and switching regulators. In a typical linear regulator, the output voltage is regulated by adjusting a passive element (e.g., a variable resistor) to control a continuous flow of current from the voltage source to the load.
Switching regulators, on the other hand, are essentially DC-DC converters that operate by switching current ON and OFF to control the output voltage. Switching voltage regulators typically employ one or more switching devices, along with an inductor and a capacitor in order to store and transfer energy to a load. These regulators are able to regulate the voltage being supplied to the load by turning the switching element(s) ON and OFF, thereby controlling the amount of power being transmitted through the inductor in the form of discrete current pulses. The inductor and the capacitor convert the supplied current pulses into a steady load current so that the load voltage is regulated. Ultimately, regulation of the output voltage is achieved through adjustment of the switch ON-OFF timings based on feedback signals indicative of the output voltage and load current.
Switching regulators that operate in current-mode are particularly desirable. They provide good line and load transient signal rejection, and possess inherent current-limiting capabilities during fault conditions (e.g., output short circuits). Current-mode switching regulators typically monitor the inductor current and compare it with a peak inductor current level to determine when it is appropriate to turn OFF the main switching element, thereby eliminating the supply of excess current.
Normally, current-mode switching regulator circuits include the following: a logic section, an output switch or switches controlled by the logic section, an oscillator for providing periodic timing signals to turn ON the main switch, a current amplifier that relays a sense voltage that is dependent on the inductor current, an error amplifier that adjusts its output voltage depending on load conditions, and a current comparator producing a signal which causes the logic section to turn OFF the main switch when the sense voltage increases above the voltage emerging from the error amplifier.
A particular type of regulator which often operates in current-mode as described above is the synchronous switching regulator. These regulators have a main switching element and a synchronous switching element which are driven out of phase with respect to each other in order to supply current at a regulated voltage to a load. Synchronous switching regulators differ from non-synchronous switching regulators in that a diode is replaced with a synchronous switching element, and the result, typically, is decreased power loss in the switching regulator.
A major benefit of switching regulators, such as synchronous switching regulators, is that they typically exhibit greater efficiency (where efficiency is defined as the ratio of the power provided by the regulator to the power provided to the regulator) than can be found in linear regulators, thereby leading to significant reductions in unwanted heat dissipation. As a result, many switching regulators can eliminate the use of a heat sink that an equivalent linear design would require.
In particular, synchronous switching regulators that employ MOSFET (metal-oxide semiconductor field-effect transistor) switches are widely used in portable battery-powered electronic products and products in which only limited heat generation can be tolerated. Because these voltage regulators exhibit higher efficiency, they provide relatively long battery life with little heat generation. For this reason, these regulators are often employed in systems such as cellular telephones, cordless telephones, personal pagers, laptop computers, and wireless modems.
The efficiency of switching regulators, however, is not always maximized and varies proportionally to the size of the load. It is a function of output current and typically decreases when the switching regulator is providing small amounts of current to the load. This occurs because even as the load decreases, a fixed amount of power is dissipated in the drive circuitry irrespective of the load size.
The above described loss of efficiency at lighter loads is common in switching regulators that operate in a forced continuous mode of operation. In forced continuous mode, the efficiency loss at lighter loads for switching regulators becomes greater because the main switch is periodically turned ON and OFF regardless of operating conditions. Therefore, these regulators may become inefficient for smaller loads because of the energy, in the form of gate charge, that is required to constantly turn the main switch and synchronous switch ON and OFF regardless of load conditions.
An effective alternative to operating in forced continuous mode is to allow the regulator to enter Burst Mode operation. When operating in this mode, the regulator may omit switching cycles when the load is light, thereby reducing transistor gate charge losses. This is possible because, when operating in Burst Mode, the switching transistor(s) and other unneeded portions of the regulator circuit are made to remain OFF as the load current drops below a specified value. This technique is therefore used to reduce switching losses in a switching regulator and increase the operating efficiency at low output current levels.
Available regulators capable of operating in Burst Mode use essentially the same circuitry as described above for typical switching regulators, with the addition of a burst comparator and circuitry that provides a burst threshold level. This additional circuitry may be used to shut down the majority of a regulator circuit under specified conditions in order to reduce power consumption. Examples of regulators that use Burst Mode and forced continuous mode are Linear Technology Corp.'s LTC1435 and LTC1735 series products.
A disadvantage of prior Burst Mode type regulators results from the inability to externally control the burst threshold level, which sets the minimum peak inductor current level (the level below which the circuit enters into Burst Mode). For higher burst threshold levels, the efficiency at light loads is higher at the expense of higher output voltage ripple (an undesirable characteristic). For lower burst threshold levels, the output voltage ripple is lower at the expense of slightly reduced efficiency for light loads. Accordingly, because the burst threshold level, which sets the minimum peak inductor current level, is fixed internally in current regulators that operate in Burst Mode, the output voltage ripple and efficiency of a regulator cannot be tailored to fit the requirements of different applications.
In view of the foregoing, it would be desirable to provide a circuit and method for varying the peak inductor current level for Burst Mode in current-mode DC-DC converters to optimize efficiency of a regulator, and to allow the voltage ripple of the regulated output voltage to be adjusted over a continuous range of values.